Private premise-based telephone systems, such as those installed at correctional facilities, have created a need to monitor various events occurring on the telephone lines of the system. Telephone systems at correctional facilities may comprise a microprocessor-based call processing system having operational software that is capable of allowing control over telephones connected to the system. For example, the system may be programmed to prevent inmates from contacting unauthorized parties or using the telephone system for fraudulent purposes. An authorization mechanism may be utilized to prevent inmates from dialing unauthorized numbers directly.
A particular problem that is encountered with these systems is the placement of a three-way call by a party that is authorized to be called by the inmate. Once the inmate is connected to an authorized number, the inmate may be connected to a third party at an unauthorized number via the three-way call feature.
A three way call is initiated when the originally called party (an authorized party outside the private telephone system) depresses the hook switch on the telephone, generating a hook flash signal. This signals the telephone central office to put the inmate on hold and provide a dial tone to the originally called party. On receipt of the dial tone, the originally-called party dials the number of an unauthorized third party, and when the connection is completed, the inmate and the unauthorized third party can communicate through the connection established outside the private system.
Conventional three-way call monitoring systems rely on the detection of telephone signals. They typically monitor the local telephone connection for the hook flash “click” signal or associated central office signals that fall in a frequency band outside the range of signal frequencies produced by the human voice. These systems typically monitor signals on the local telephone line through a frequency filter designed to pass audio signals in this frequency band. A three way call attempt is indicated whenever signals in the frequency band have energies above a selected threshold. Some systems even compare the signals with a hook flash reference signal utilizing sampling techniques implemented with a digital signal processor (DSP).
All of these systems are not very accurate for a number of reasons. The underlying assumptions about the frequency profile of three way call events, i.e. the hook flash and signals generated by activating central office switches, are often wrong. For example, the hook flash signals are often modified by transmission through switches and along loaded lines, and even if assumptions about the frequency characteristics of the initial signal are accurate, these characteristics may be substantially distorted by the time the “hook flash” signal reaches the three way call detection system.